1. Field of the Invention
The present invention relates to a semiconductor device that has at least one of a drain and a source buried in a semiconductor layer. More specifically, the present invention relates to a solid state image sensor that have at least one of a drain region and a photoelectric conversion region buried in the semiconductor layer, away from a surface of the semiconductor layer.
2. Discussion of the Background
An amplifying MOS image sensor (CMOS sensor) is adapted to function as a solid state image sensor. One advantage of the amplifying MOS image sensor is the low power consumption. A conventional amplifying MOS image sensor is shown in Japanese patent No. 3484071.
In the conventional amplifying MOS image sensor, a drain region is formed in a surface of a semiconductor layer or a well, such that an upper edge of the drain region coincides with an upper surface of the semiconductor layer or well.
However, there are various crystal defects that are produced on the surface and the neighboring of the surface of the semiconductor layer or well, due to the forming process of the semiconductor device. These crystal defects are known by the one of ordinary skill in the art and a size of the neighboring of the surface of the semiconductor layer that is affected by the crystal defects due to the manufacturing of the device is also known in the art. Further, the amount of defects at the surface of the semiconductor layer is larger compared to a region inside the semiconductor layer. This difference in the amount of defects is determined by the damage to the surface of the semiconductor layer during manufacturing, for example when an interlayer dielectric is deposited on the surface of the semiconductor layer or when a drain region is formed inside the semiconductor layer. Thus, charges from a photoelectric conversion region or a photodiode (PD) formed in the semiconductor layer are trapped by the crystal defects, both inside the layer and at the surface of the semiconductor layer causing a leak current to occur from or into the drain region. For simplicity, the crystal defects inside the semiconductor layer are simply called crystal defects, and the defects in the surface of the same layer are called surface states.
Furthermore, some of the charges which are transferred to the drain region are trapped in the surface states or the crystal defects which exist in the semiconductor layer above the drain region, making less likely a discharge of the charges stored in the photoelectric conversion region. Thus, a S/N ratio (signal/noise ratio) of the conventional amplifying MOS image sensor is decreased, resulting in a poor performance.